Fatty acid alkyl esters are currently used in numerous uses, such as diesel fuels, household fuels, ecological solvents, starting compounds for the manufacturing of sulfonates of fatty alcohols, of amides, of dimers of esters, etc.
In particular, one of the major uses of fatty acid alkyl esters relates to the production of biodiesel intended to be incorporated into diesel fuel. Biodiesel is in particular a biofuel of agricultural origin that forms an alternative to the usual fuel of fossil origin.
In a known manner, biodiesel or fatty acid alkyl esters are the product of the transesterification reaction between triglycerides (TG) and a monoalcohol, such as methanol, in the presence of a homogenous or heterogeneous catalyst. A second product, glycerol, is also produced during this reaction. According to the nature of the oil used at the beginning, the glycerol can represent from 10 to 15%, by weight, of the products formed. This glycerol can be reused in various uses, but must first be purified (elimination metals, salts, water).
The transesterification reaction can be represented in the following manner when the starting alcohol is methanol:

wherein R1, R2 and R3 correspond to hydrocarbon chains of fatty acids.
Thus, the molecule of triglyceride is reacted with three molecules of methyl alcohol (or methanol) in order to produce one molecule of glycerol (liquid trialcohol giving fatty acids via transesterification) and three acids called methyl esters (crude biodiesel). Among the sources of triglycerides are vegetable or animal oils and used cooking oils.
Typically, the monoalcohol can be chosen from methanol, ethanol, or butanol; methanol being, however, the most widespread reactive alcohol due to its low price.
Then, the crude biodiesel obtained is refined, then used as a fuel.
For example, in France, according to the decrees from 30 Jun. 2014 and 31 Dec. 2014, fatty acid methyl esters or FAME can be incorporated at most at 8% by volume of diesel. The requirements are in particular defined in the standard NF EN 14214:2013. Thus, the biodiesel must contain few diglycerides (maximum concentration of 0.20 mol %), monoglycerides (maximum concentration of 0.20 mol %) and little free glycerol (maximum concentration of 0.02 mol %). Moreover, in order to reinforce the requirements relative to the cold resistance of the diesels based on biodiesel, the concentration of saturated esters must be, by weight with respect to the total weight, less than 30% in the summer and than 16% in the winter. Indeed, the greater the concentration of saturated esters in a fuel, the poorer its cold resistance.
However, the chemical reaction of transesterification described above has two disadvantages: it is slow and in equilibrium, namely when the oil and the alcohol have been transformed into biodiesel and glycerol, the biodiesel and the glycerol can once again be transformed into alcohol and into oil.
This is why, in order to overcome these disadvantages, it is generally considered necessary to use a catalyst and to work under pressure and at a high temperature. Likewise, one way to push the conversion of the oil towards the formation of alkyl (methyl) ester is to work with a large excess of monoalcohol with respect to the stoichiometry and to carry out two reaction steps between which the glycerol produced is eliminated in such a way as to move the equilibrium towards the production of alkyl ester.
Numerous methods for manufacturing fatty acid alkyl esters continuously or discontinuously have already been developed in the prior art.
A first approach involves using the conventional pathways of homogenous catalysis with catalysts soluble in the reaction medium, such as soda or sodium methylate, by reacting a neutral oil and an alcohol (methanol). A representative example of this type of method is the method described in the patent application EP-0.523.767, with continuous implementation of a basic homogenous catalyst.
This type of method, however, has several disadvantages. Once the reaction has ended, the excess of homogenous catalyst present substantially in the glycerine phase in the form of alcoholates and of soaps must be neutralised, then the water and the monoalcohol (methanol) must be eliminated via evaporation. The evaporated monoalcohol (methanol) must often be distilled. For the ester fraction, the traces of alkaline compounds are eliminated by washing with water and drying.
The document U.S. Pat. No. 7,145,026 describes a method for manufacturing fatty acid methyl esters continuously at a temperature ranging from 80 to 180° C. also using a homogenous catalyst. In particular, according to a specific embodiment, this method involves in particular reacting the methanol pre-mixed with a caustic catalyst, such as NaOH, with TGs in a plug flow reactor formed by wound tubes of copper for example of approximately 2 metres (in general ranging from 3 to 9 m for an average diameter of 15 cm) for a residence time ranging from 9 to 16 seconds and a temperature ranging from 50° C. to 87° C. The more the temperature is increased, the greater the rate of conversion into esters: it is for example 78% for a residence time of 9 seconds at 86.8° C. and 43% for a residence time of 9 seconds at 65° C. It is also indicated that the conversion rate is increased by increasing the pressure, for example from 1 bars to 15 bars or the residence time. The increase in temperature and in pressure can be carried out in suitable devices placed upstream of the reactor. In order to obtain fatty acid methyl esters, the products of the reaction (glycerol, catalyst, methyl esters) are separated in a separator provided for this purpose (separation of the products by heating, then the methyl esters are washed with water).
Consequently, even if the conversion rates are relatively high, they remain far from the optimal levels of 90% or more, and this method also has the disadvantage of having to eliminate the catalyst from the reaction products. Moreover, it requires steps of heating and of increasing the pressure that complicate the implementation of this method.
A second approach implements heterogenous catalysts, namely insoluble in the reaction medium.
The document EP 0 924 185 describes in particular a method for manufacturing alkyl esters derived from vegetable oils comprising three steps:                a first step (a) involves reacting a vegetable oil with an excess of monoalcohol in the presence of a heterogenous catalyst, followed by an elimination of the monoalcohol in excess and a separation of the glycerine, in such a way as to obtain a crude ester containing residual monoglycerides;        a second step (b), in which the crude ester thus obtained is subjected to a reaction of re-esterification of the residual mono-glycerides into di- and tri-glycerides, in the presence of a heterogenous catalyst; and        a third step (c), in which an evaporation of the ester at a reduced pressure is carried out with recycling of the evaporation residue into the starting oil of step (a).        
The main disadvantage of such a method is the very high economic cost represented by the vacuum distillation of the entire production. Moreover, the presence of recycling also represents an additional cost. Finally, experiments show that even at a highly reduced pressure, the bottom temperature of the column for evaporation of the ester is significant, which leads to a serious risk of degradation of the residue. The latter cannot therefore be totally recycled, it must periodically be purged, which has a negative effect on the yield of the method.
The document FR 2 838 433 also describes a method for producing fatty acid alkyl esters and glycerol in the presence of a heterogeneous catalyst, zinc aluminate. This method involves a succession of three balanced reactions occurring in parallel:                an oil, such as a colza, palm, sunflower, etc. oil (representing 20 to 80%, preferably 45 to 55% by weight) is reacted with a molecule of monoalcohol, such as methanol, in order to give a molecule of alkyl ester and a diglyceride;        the diglyceride thus obtained (representing 20 to 80%, preferably 45 to 55% by weight) is reacted with a molecule monoalcohol (methanol) to give a molecule of alkyl (methyl) ester and a monoglyceride;        the monoglyceride thus obtained is reacted with a molecule of monoalcohol (methanol) to give a molecule of alkyl (methyl) ester and a molecule of glycerine.        
The reaction is carried out in general in a plurality of successive fixed-bed reactors operating with an ascending flow and in liquid phase, each of the reactors being supplied with a mixture of colza oil and of monoalcohol (methanol) (first reactor) or for the most part of alkyl (methyl) ester and of monoalcohol (methanol) (second reactor and optionally the following reactors). The reactions are carried out at a temperature ranging from 180° C. to 220° C. at a pressure of 30 bars to 80 bars. Between the reactions, the excess monoalcohol is eliminated by evaporation in order to be recycled and the product obtained is decanted in a settling tank in order to separate the phases rich in alkyl ester from the phase rich in glycerol.
At the reactor outlet, the methyl ester and a joint product of the reaction, glycerol, as well as the excess methanol, are obtained.
Consequently, this method has the disadvantage of being very complicated, of requiring the use of numerous devices positioned successively (reactors/evaporators/condensers/settling tanks, etc.). Moreover, it requires working at high temperatures (180° C. to 220° C.) and at high pressures (30 bars to 80 bars).
The document US 2007/0260079 describes a method for manufacturing biodiesel that can use a homogenous catalyst, a heterogenous catalyst or both.
In particular, the method involves:                reacting the TGs, the alcohol (methanol), the catalyst in a first reaction zone formed by a mixer followed by a tubular reactor (residence time 20 to 30 seconds at a pressure of 3.5 to 20 bars and a temperature ranging from 70 to 200° C.) in order to form a first intermediate mixture that comprises the product transesterified fatty acid ester, the alcohol that has not reacted, the TGs that have not reacted or have partially reacted;        eliminating the glycerol and the alcohol that has not reacted from this intermediate mixture in particular via a hydrocyclone that allows to separate the heavier phase, namely the glycerol, from the fatty acid methyl esters formed comprised in a second intermediate mixture;        bringing this second intermediate mixture to a second reaction zone also comprising a mixer followed by a tubular reactor:        adding, to this second reaction zone, alcohol and the catalyst (in the mixer) in order to form a third intermediate mixture which, once brought to the reactor (residence time of approximately 30 to 60 seconds at a pressure of 10 to 27.5 bars and a temperature ranging from 70 to 200° C.), allows to form fatty acid alkyl esters and glycerol;        separating the alcohol, the glycerol and the fatty acid alkyl esters via a hydrocyclone.        
Consequently, this method also has the disadvantage of requiring the use of numerous devices (mixer/reactor and hydrocyclone) positioned successively in order to carry out the reaction of transesterification and requiring steps of heating and of pressurising.
The document US 2015/0336067, which describes the manufacturing of biodiesel from a vegetable oil, methanol and a catalyst, is also known from the prior art. These raw materials are placed in a tank that is connected to a vibration device that allowed the reaction of transesterification.
In summary, to manufacture biodiesel, it is currently necessary to use such numerous steps that only large units are in general capable of being economically profitable.
There is therefore a real need for a new method for manufacturing fatty acid alkyl esters that would allow the manufacturing of such compounds simply and economically, for example by having a smaller number of steps, by using less implementation devices (reactor, settling tank, evaporator, condenser, etc. placed successively), while being fast (reduced residence time).
There is also a need to provide a new method in which the manufacturing of these fatty acid alkyl esters does not require any particular heating or pressurising that could increase the manufacturing costs.
The goal of the present invention is consequently to propose a new method for manufacturing fatty acid alkyl esters at least partly avoiding the aforementioned disadvantages.